The present invention relates generally to systems for transmitting carrier chrominance signals of color video signals, and more particularly to systems capable of transmitting, with a good S/N ratio, carrier chrominance signals of color video signals in which there is a limit to the carrier chrominance signal transmission level.
In general, a signal transmission system for recording and reproducing color video signals on and from a recording medium such as a magnetic tape or a rotary disc, has a limit for the transmissible level of the carrier chrominance signal. For example, in an apparatus for recording and reproducing a color video signal on and from a magnetic tape, use is made of a system which records on the magnetic tape by separating the color video signal into a carrier chrominance signal and a luminance signal, frequency-modulating the luminance signal, frequency-converting the carrier chrominance signal to a frequency band which is lower than the band of the frequency-modulated luminance signal, multiplexing this frequency-modulated luminance signal and this frequency-converted carrier chrominance signal, and then recording this multiplexed signal on the magnetic tape.
If this recording system uses a high level of the frequency-converted carrier chrominance signal multiplexed with the frequency-modulated luminance signal, the transmission causes deterioration of the S/N ratio of the carrier chrominance signal decreases. However, when the transmission characteristic, the effect on the luminance signal, etc., and like factors are duly taken into consideration, the optimum level of the carrier chrominance signal which can be transmitted is determined. Accordingly, there is also a limit to improving the S/N ratio by merely increasing the level of the carrier chrominance signal.
In general, in a color video signal of the NTSC system, the level of the color burst signal is 6 dB lower than the peak value of the carrier chrominance signal component representing color bars and the like. In this connection, it should be understood that the term "carrier chrominance signal" herein refers collectively to the color burst signal and the carrier chrominance signal component. Therefore, the term "carrier chrominance signal component" designates that part of the carrier chrominance signal which is exclusive of the color burst signal.
Heretofore, the carrier chrominance signal has been transmitted with the level of the carrier chrominance signal component used as it is, and the level of only the color burst signal increased by an increment in the order of 6 dB, up to the optimum level at which transmission is possible. By this method, the color burst signal can be transmitted with a good S/N ratio. The jitter correction is made at the time when the frequency of the converted carrier chrominance signal is returned to its original frequency. As a comprehensive result, the S/N ratio of the carrier chrominance signal increases.
In the above described prior system, however, the level of the carrier chrominance signal component is not high. For this reason, the color image is pale or light. When the level of the carrier chrominance signal is low, the S/N ratio of the carrier chrominance signal component itself is disadvantageously impaired by the transmission.
In an ordinary color video signal, except in the case of a test signal of a color bar or the like, the level of the carrier chrominance signal component is relatively low and is not always the optimum transmissible level of the transmission system.
Accordingly, with this point of view, the present invention contemplates transmission with an increased level of the carrier chrominance signal component within the possible range to increase the S/N ratio of the carrier chrominance signal.